Result is -Number
which should be preferred for portability.
Number and Result have to be of the same type.
In coroutining mode, if Number is uninstantiated, the call to -/2 is delayed until this variable is instantiated.
Success:
-(1, -1).
-(5, Result). (gives Result = -5)
-(-6.2, Result). (gives Result = 6.2)
Fail:
-(1, 0).
-(1, -1.0).
-(1.0, -1).
-(1, r).
Error:
-(A, 6). (Error 4).
-(4 + 2, -6). (Error 24).